1. Field of the Invention
The present invention relates to an optical system and an image pickup apparatus having the same, and is suitable for, for example, silver-halide film cameras, digital still cameras, and video cameras.
2. Description of the Related Art
Many lenses, filters, and so forth formed of resin such as acrylic resin or polycarbonate resin are used in optical systems used in recent image pickup apparatuses such as digital cameras and video cameras.
Optical elements formed of resin are characterized by high moldability and high shape freedom. In addition, adhesion using thermoplasticity, ultraviolet curability, and so forth of resin is possible. It is also possible to transfer a fine structure onto the surface of an optical element. Therefore, optical elements formed of resin are widely used in optical systems, as plastic mold lenses, adhesive layers bonding surfaces together, diffractive optical elements, focusing plates, microlenses for image sensors, color filters, and so forth.
Optical surfaces of optical elements are often provided with an antireflection structure. However, it is technically difficult to deposit a dielectric antireflection coating onto an optical surface of an optical element formed of resin. Therefore, instead of depositing a dielectric antireflection multilayer coating onto an optical surface of an optical element formed of resin, there is proposed to form a fine textured periodic structure (depressions or protrusions) shorter than the wavelengths of visible light to obtain an antireflection effect. Japanese Patent Laid-Open No. 2005-157119 discloses an optical element having a fine textured periodic structure formed on an optical surface thereof. Japanese Patent Laid-Open No. 2006-10831 discloses an antireflection structure in which fine protrusions or depressions are staggered.
Resin is a macromolecule made of carbon atoms bonded together. Incidence of short-wavelength light breaks the bond in the macromolecule and causes physical and chemical changes. The bond energy and the wavelength of light corresponding to the bond energy are fixed. For example, the bond energy of C—C bonds is 346 kJ/mol, and the wavelength λ of light corresponding thereto is 347.2 nm. The bond energy of C═C bonds is 340 kJ/mol, and the wavelength λ of light corresponding thereto is 353.3 nm. The bond energy of C—O bonds is 386 kJ/mol, and the wavelength λ of light corresponding thereto is 311.2 nm. The bond energy of C═O bonds is 374 kJ/mol, and the wavelength λ of light corresponding thereto is 321.2 nm. Elemental sulfur S is used to increase the refractive index of resin. Since the bond energy of C—S bonds is low (269 kJ/mol), C—S bonds are easily broken, and resin can become yellow due to the effects of elemental sulfur S.
As described above, braking of these bonds by short-wavelength light (ultraviolet light of 300 nm to 400 nm) is one of the causes of deterioration in optical performance when optical elements formed of resin are used.
In general, lens materials have high absorption coefficients for ultraviolet light. Reflection-reducing multilayer coatings for visible light have low transmittances for ultraviolet light. Therefore, commonly used optical systems have low transmittances for ultraviolet light. Therefore, optical members formed of resin disposed on the image side of an optical system are irradiated with a relatively small quantity of ultraviolet light. Therefore, the effects of incidence of ultraviolet light on the optical elements formed of resin should not be significant. However, if converging ultraviolet light falls on the optical elements formed of resin, the effects of the ultraviolet light is not negligible.
Antireflection multilayer coatings used in lens systems have high antireflection properties for visible light but function as reflectance-increasing coatings for ultraviolet light.
FIG. 13 illustrates the spectral property of a commonly used antireflection coating. Commonly used antireflection multilayer coatings have increased reflectances in the wavelength regions of ultraviolet light and infrared light when they are optimized so as to have reduced reflectances throughout the visible light range. Antireflection multilayer coatings are made by layering a dielectric layer having a low refractive index (L-layer) and a dielectric layer having a high refractive index (H-layer). The layers have a thickness such that the light path length is about quarter of the wavelength. Reflections undergo destructive interference. However, in the cases of ultraviolet light and infrared light, reflections undergo constructive interference.
FIGS. 7 and 8 are schematic sectional views of an image pickup apparatus and an optical system commonly used therein.
Reference numeral 1 denotes an interchangeable lens (photographing optical system). Reference numeral 2 denotes a single-lens reflex camera, to which the photographing optical system 1 is detachably attached. Reference letter Gi denotes the ith lens constituting the photographing optical system 1 counting from the object side (light incidence side). Reference numerals 3 and 4 denote the object side and image side optical surfaces, respectively, of the first lens G1. Reference numerals 5 and 6 denote the object side and image side optical surfaces, respectively, of the second lens G2. Reference numerals 7 and 8 denote the object side and image side optical surfaces, respectively, of the third lens G3.
The fourth lens G4 is a compound aspheric lens. On the object side optical surface 9, an aspheric layer 9a is formed by molding of plastic resin. A lens element 9b having optical surfaces 9 and 10 is formed of a normal glass lens material. The optical surfaces 9 and 10 are spherical surfaces. Reference numeral 11 denotes an image pickup element disposed on the optical axis of the optical system 1 in the single-lens reflex camera 2. Reference numeral 12 denotes a light source. Reference numeral 13 denotes a finder.
In FIG. 7, ultraviolet light from an intense point light source 12 such as the sun passes through the optical surfaces 3 and 4 of the first lens G1 and then passes through the optical surfaces 5 and 6 of the second lens G2. The ultraviolet light can be reflected by the optical surface 7 of the third lens G3 and then by an antireflection multilayer coating on the optical surface 6 of the second lens G2. As a result, the ultraviolet light can fall on the optical element 9a formed of resin as a surface reflection ghost image.
As shown in FIG. 8, when some of the lens elements of the photographing optical system 1 are at certain positions due to zooming or focusing, ultraviolet light from the light source 12 outside the field of view can fall on the image pickup element 11 as a ghost image.
Since the art of Japanese Patent Laid-Open No. 2005-157119 uses a fine textured periodic structure, the reflectance for visible light can be reduced but many diffraction rays are generated for ultraviolet light.
For example, as shown in FIG. 9, even if the light source 12 is outside the field of view, incident light is significantly diffracted by the fine textured periodic structure formed on the optical surface 6 and can reach the image pickup element 11 via the rear optical system.
In the art of Japanese Patent Laid-Open No. 2005-157119, the fine textured periodic structure is a two-dimensional square array on a surface of a lens. Therefore, diffraction rays tend to concentrate in the lattice direction.
Also in the antireflection element of Japanese Patent Laid-Open No. 2006-10831, as in the art of Japanese Patent Laid-Open No. 2005-157119, diffraction rays are generated by a periodic structure. The two-dimensional extent of diffraction rays is, unlike in the case of the square array, in oblique directions or six directions at intervals of 60 degrees. However, the number of rays into which a ray is split is the same. Therefore, the effect of dispersing the energy of a ray is insignificant. Therefore, also in this case, ultraviolet light is strongly diffracted in certain directions.
As described above, in the cases of the conventional antireflection structures, although the quantity of ultraviolet light with which an entire optical element formed of resin is irradiated is small, converging ultraviolet light can locally deteriorate the optical element.